The ?4 subunit of the ?6?4 integrin, which forms hemidesmosomes in quiescent normal cells, becomes phosphorylated in breast tumor cells that overexpress HER2 or EGFR. This phosphorylation converts the cytoplasmic domain of the integrin into a signaling scaffold that drives cell invasion, proliferation and survival. HER2 and EGFR are expressed in HER2+/ER- breast cancer, and EGFR is overexpressed in the triple- negative (HER2-,ER-,PR-) subtype. Both cancers are highly aggressive and resist treatments currently available in the clinic. Because these cancer types often overexpress the ?6?4 integrin as well, we have now examined their dependence on signaling from these receptor complexes. We have discovered that these signaling mechanisms are essential for the growth and survival on the cancer cells, and that their signaling requires the assembly of the integrin and HER2 or EGFR with syndecans, another family of matrix receptors. Indeed, syndecan-1 appears necessary for signaling by HER2??6?4, and syndecan-4 appears to be required by EGFR??6?4. Our goal is to define the molecular details of syndecan assembly with these signaling complexes, develop mutants and blocking peptides that disrupt the organizing function of these two syndecans, and test the mutants and peptides in tumor growth, angiogenesis and the activity of cancer stem cells in animal models of HER2+ and TN breast cancer. The outcome of this work will provide potential insight into the development of new therapeutics to target HER2+ and TN breast cancer.